File:Phanerozoic Climate Change Rev.png

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Description

Expansion showing climate change during the last 65 million years. Note that the scales are not numerically the same since they are based on measurement of different types of taxa under different conditions.

This figure shows the long-term evolution of oxygenisotope ratios during the Phanerozoiceon as measured in fossils, reported by Veizer et al. (1999), and updated online in 2004 [1]. Such ratios reflect both the local temperature at the site of deposition and global changes associated with the extent of continental glaciation. As such, relative changes in oxygen isotope ratios can be interpreted as rough changes in climate. Quantitative conversion between this data and direct temperature changes is a complicated process subject to many systematic uncertainties, however it is estimated that each 1 part per thousand change in δ18O represents roughly a 1.5-2 °C change in tropical sea surface temperatures (Veizer et al. 2000).

Also shown on this figure are blue bars showing periods when geological criteria (Frakes et al. 1992) indicate cold temperatures and glaciation as reported by Veizer et al. (2000). The Jurassic-Cretaceous period, plotted as a lighter blue bar, was interpreted as a "cool" period on geological grounds, but the configuration of continents at that time appears to have prevented the formation of large scale ice sheets.

All data presented here have been adjusted to the 2004 ICSgeologic timescale[2]. The "short-term average" was constructed by applying a σ = 3 Myr Gaussian weighted moving average to the original 16,692 reported measurements. The gray bar is the associated 95% statistical uncertainty in the moving average. The "low frequency mode" is determined by applied a band-pass filter to the short-term averages in order to select fluctuations on timescales of 60 Myr or greater.

On geologic time scales, the largest shift in oxygen isotope ratios is due to the slow radiogenic evolution of the mantle. A variety of proposals exist for dealing with this, and are subject to a variety of systematic biases, but the most common approach is simply to suppress long-term trends in the record. This approach was applied in this case by subtracting a quadratic polynomial fit to the short-term averages. As a result, it is not possible to draw any conclusion about very long-term (>200 Myr) changes in temperatures from this data alone. However, it is usually believed that temperatures during the present cold period and during the Cretaceous thermal maximum are not greatly different from cold and hot periods during most of the rest the Phanerozoic. Some recent work has disputed this (Royer et al. 2004) suggesting instead that the highs and lows in the early part of the Phanerozoic were both significantly warmer than their recent counterparts.

Common symbols for geologic periods are plotted at the top and bottom of the figure for reference.

Long-term evolution

The long-term changes in isotope ratios have been interpreted as a ~140 Myr quasi-periodicty in global climate (Veizer et al. 2000) and some authors (Shaviv and Veizer 2003) have interpreted this periodicity as being driven by the solar system's motions about the galaxy. Encounters with galactic spiral arms can plausibly lead to a factor of 3 increase in cosmic ray flux. Since cosmic rays are the primary source of ionization in the troposphere, these events can plausibly impact global climate. A major limitation of this theory is that existing measurements can only poorly constrain the timing of encounters with the spiral arms.

The more traditional view is that long-term changes in global climate are controlled by geologic forces, and in particular, changes in the configuration of continents as a result of plate tectonics.

Temperature Record Series

This figure is part of series of plots showing changes in Earth's temperature over time.

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Notes

The statistical errors, plotted in gray, may be significantly smaller than the systematic biases that can occur. Such systematic concerns include:

Different fossil types, spanning different phyla, are measured in different parts of the record, and biological differences in how oxygen is incorporated into different fossils may introduce biases.

Because oxygen isotopes reflect both local temperatures and global glaciation, it is necessary to sample from large spatial areas to provide adequate global coverage. Such extensive coverage may not be available in all time periods (particularly older periods).

In constructing the "low frequency mode", the filter was applied to the short-term averages rather than to the data directly because applying such a filter to the data directly strongly biases the results towards the value at times that have been heavily sampled.